Fix minor corrections in setup guide

[arpitpatawat]

Issue #, if available:
Nil
Description of changes:

1. Added Amazon Root CA certificate which also needs to be configured, in the Setup readme.
2. Reordered the sequence of parameters which needs to be configured, based on docs/examples/sample_nucleus_config.yaml
3. Renamed docs/examples/sample_nucleus_config.yml -> docs/examples/sample_nucleus_config.yaml since later, .yaml is being used, and it might create a confusion for yml and yaml, although they are the same - (https://stackoverflow.com/questions/22268952/what-is-the-difference-between-yaml-and-yml-extension)
4. Added instructions to copy the sample configuration as config.yaml rather than init_config.yml, since later it is being copied as config.yaml.
5. Changed the example config path display from doc/... to docs/....

PS: Please let me know if there was any plan for having .yml extension for the sample configuration, just so that this PR does not break anything (although it does not make much sense since later it was changed to .yaml). Additionally, I have not run and tested the service completely, just updated these minor changes. Please have this in mind while reviewing. Thank you.

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